1. Field of the Invention
The present invention relates to a laser beam composite apparatus for use in the optical systems of image forming apparatus.
2. Description of the Related Art
Image forming apparatus of the electrophotographic type such as is used in digital type copying apparatus, laser printers and the like, scan the surface of a photosensitive member by exposing said surface via laser beams modulated in accordance with image signals and deflected by polygonal mirrors.
When the aforementioned apparatus form multicolor images, a plurality of laser beams corresponding to the respective colors are used to expose the surface of a single photosensitive member at different positions thereon, or to expose the surfaces of a plurality of photosensitive members.
When a plurality of individual laser beams are deflected using a plurality of polygonal mirrors, the optical system becomes more complex and larger in size, and relative positional dislocation of the various colors readily occurs.
Accordingly, optical systems used for forming multicolor images generally are constructed so as to combine a plurality of laser beams into a single composite beam, deflect said single beam via a single polygonal mirror, and thereafter split said single composite beam into the original plurality of beams which are guided to their respective exposure positions.
Conventional laser beam combining methods include well known methods for combining two beams which have been split by a beam splitter so as to have mutually dissimilar polarization directions, and methods using dichroic mirrors having wavelength selectivity so as to combine a plurality of beams having mutually dissimilar wavelengths into a single beam. However, methods which combine beams of mutually dissimilar polarization directions are limited to combining two beams. Furthermore, although methods which combine beams of dissimilar wavelengths can combine three or more beams, the wavelength range (frequency band range) of the composite beam increases as the number of combined beams increases, such that certain disadvantages arise inasmuch as excellent multicolor images cannot be obtained due to the optical characteristics (color aberration and the like) of the lens system through which the composite beam passes and the spectral sensitivity characteristics of the photosensitive member.